The Dow has made its biggest point gains on Tuesday's this year, rising for 20 straight Tuesday's.

The Dow Jones industrial average has posted gains every Tuesday for the past 20 weeks.

That makes it the longest winning streak for any day of the week since 1900, according to Ryan Detrick, chief technical analyst at Schaeffer's Investment Research. The previous record was 13 (a three-way tie between Monday, Wednesday and Friday).

The bulk of the Dow's (DJIA) gains this year were reached on Tuesdays. Detrick said the index has added a total of 1,518 points during the 21 Tuesdays that the market was open in 2013.

While it could be a fluke, the run begs the question: Why Tuesday?

Here are some of the leading theories making the rounds on Wall Street, courtesy of David Lutz, head ETF trader at Stifel Nicolaus.

It's random. There may be no explanation. Tuesday is just a day, like any other. That said, the trend has been remarkably consistent over a long period of time.

POMO. The Permanent Open Market Operation, or POMO, is the official name for the Federal Reserve's bond buying program.

Tuesdays and Fridays have been among the largest purchase days of the week, and there tends to be an upward bias in the stock market on days when the Fed is buying bonds.

Mutual funds. Investors have poured billions of dollars into stock-based mutual funds this year, with an average of $3 billion coming into the market each week.

Many mutual fund investors, who typically don't trade for a living, place orders to buy or sell stocks over the weekend. But the orders need to be cleared and the funds usually aren't put to work until late Monday or early Tuesday.